1. Field of the Invention
The present invention relates to a process of forming an electrode-to-electrode bond structure. More specifically, the present invention relates to a process of forming an electrode-to-electrode bond structure which can be applied to e.g. bonding as well as electrically connecting a semiconductor chip to another semiconductor chip, mounting a semiconductor chip on a wiring board, and connecting a wiring board to another wiring board.
2. Description of Related Art
There is a growing demand in recent years for increased density in mounting of electronic parts on e.g. a printed wiring board and a ceramic substrate. As away for satisfying such a demand, a bear-chip mounting method is attracting attention. In the bear-chip mounting method, conventional face-up mounting is being taken over by a face-down mounting, i.e. flip chip bonding. In the face-up mounting, electric connection between the semiconductor chip and the wiring board is established usually by means of wire bonding, whereas in the face-down mounting, electrical connection is established by solder bumps between the semiconductor chip and the wiring board. This technique of establishing electrical connection via the solder bumps or solder material is also applied to a connection between two separate semiconductor chips or between two separate wiring boards, as disclosed in JP-A-2-96343, JP-A-4-326747, JP-A-5-326628, JP-A-6-262386, JP-A-8-64639, JP-A-9-260059, JP-A-11-135552, JP-A-11-191673 for example.
FIGS. 6a through 6j show a conventional method for making a flip chip bonding. According to the conventional flip chip bonding method, first, as shown FIG. 6a, a metal mask 430 is prepared, in which openings 430a are formed at positions corresponding to electrodes 411 of a semiconductor 410.
Next, as shown in FIG. 6b, the metal mask 430 is placed on the semiconductor chip 410 with the openings 430a aligned with the corresponding electrodes 411.
Next, as shown in FIG. 6c, a solder paste 440 containing a predetermined solder powder is filled into the openings 430a by means of printing.
Next, as shown in FIG. 6d, the metal mask 430 is removed from the surface of the semiconductor chip 410, leaving the solder paste 440.
Next, as shown in FIG. 6e, a heating step follows for melting the solder powder in the solder paste 440 to form bumps 412 on the electrodes 411.
After the formation of the bumps 412 on the electrodes 411 of the semiconductor chip 410, a flux 450 is applied on the wiring board 420, as shown in FIG. 6f. The flux 450 serves to remove an oxide coating from the surface of the bumps 412 while preventing the bumps 412 from re-oxidizing by prohibiting contact with air during the subsequent re-flow soldering step. The flux 450 also performs an additional function of providing preliminary fixation of the semiconductor chip 410 onto the wiring board 420.
Next, as shown in FIG. 6g, the semiconductor chip 410 is placed on the wiring board 420 with electrodes 421 of the wiring board 320 aligned with the corresponding bumps 412.
Next, as shown in FIG. 6h, a heating step for re-flowing the bumps 412 follows to connect the electrodes 411 and the electrodes 421 with the bumps 412.
Next, as shown in FIG. 6i, the flux 450 is washed and removed. In this way, the flip chip bonding of the semiconductor chip 410 to the wiring board 420 is established.
Finally, as shown in FIG. 6j, an adhesive or an under-fill resin 460 is loaded between the semiconductor chip 410 and the wiring board 420. The under-fill resin 460 protects the bump 412 that serves as a conductor to connect the electrode 411 and the electrode 421 while also protecting the surface of the semiconductor chip 410 and the surface of the wiring board 420, thereby maintaining the bond reliability for along time.
However, according to the conventional bonding process described above, when the metal mask 430 is placed on the semiconductor chip 410, the openings 430a must be aligned with the electrodes 411, which becomes increasingly difficult as the electrodes 411 are disposed at a smaller pitch. In particular, when the electrodes 411 are disposed at a pitch of not greater than 200 xcexcm, the relative magnitude of an alignment error in placing the metal mask 430 becomes very large. Thus, the alignment error in the metal mask 430 results in positional error of the bumps 412 and may cause damage or loss of electric conduction in the flip chip bonding.
When the electrodes 411 are disposed at a pitch not greater than 200 xcexcm, and if the size of electrodes 412 is half the pitch, the bumps 412 formable on the electrode 411 can have a diameter of about 70 xcexcm. After bonding via the bumps 412 of such a size, the semiconductor chip 410 and the wiring board 420 is spaced by a distance not greater than 50 xcexcm. If the distance between the semiconductor chip 410 and the wiring board 420 is so small as such, it is difficult to remove the flux sufficiently in the process step of FIG. 6i. The flux remaining between the semiconductor chip 410 and the wiring board 420 can cause such problems as corrosion of the bumps 412, decrease of dielectric resistance between the electrodes, and insufficient filling of the under-fill resin 460. In addition, if the distance between the semiconductor chip 410 and the wiring board 420 is that small, voids can easily develop in the under-fill resin 460 in the process step of FIG. 6j, making it difficult to properly fill the under-fill resin 460 between the semiconductor chip 410 and the wiring board 420.
Thus, according to the conventional method, it is difficult to obtain a high bond reliability when the electrodes are disposed at a small pitch or at a high density.
Further, according to the above-described conventional method, a large number of steps including application and removal of the flux 450 and filling of the under-fill resin 460 must be performed. In other words, the process is complex.
For the purpose of simplifying the bonding process, a fluxing under-fill resin is used in recent years. The fluxing under-fill resin is an epoxy resin containing a flux as an additive, and is intended to serve as an under-fill resin as well as a flux. For example, the fluxing under-fill resin is applied on the wiring board 420 in the step of FIG. 6f, just as the flux is applied, and then heated, without being washed or removed, to harden between the semiconductor chip 410 and the wiring board 420 in the step of FIG. 6j, just like an ordinary under-fill resin 460.
The fluxing under-fill resin has to contain an inorganic filler in order to reduce its thermal expansion coefficient, thereby attaining reliability of the bond between the semiconductor chip 410 and the wiring board 420. However, if the inorganic filler is contained in the fluxing under-fill resin at a proportion of no lower than 20 wt %, such a large amount of the inorganic filler causes the fluxing under-fill resin to easily enter the boundary between each bump 412 and a corresponding electrode 421, resulting in a very sharp decrease of adhesion of the bump 412 relative to the electrode 421. For this reason, the addition of the inorganic filler to the extent of reducing the thermal expansion of the fluxing under-fill resin to a necessary level can result in an initial conduction failure caused by the poor bonding rate of the bumps. Another problem is that the fluxing under-fill resin is poor in utility because it is a single-liquid adhesive and has a short service life at room temperature.
It is, therefore, an object of the present invention to provide a process of forming an electrode-to-electrode bond structure suitable for high-density mounting, capable of achieving a sufficient reliability of the bond and achievable in a small number of steps.
Another object of the present invention is to provide an electrode-to-electrode bond structure formed by such a process.
According to a first aspect of the present invention, a process is provided for making an electrode-to-electrode bond structure. The method comprises the steps of forming a resin coating on a first bonding object having a first electrode portion for covering the first electrode portion, forming an opening in the resin coating to expose the first electrode portion, filling the opening with a metal paste containing a metal, placing the first bonding object and a second bonding object having a second electrode portion in a manner such that the metal paste filled in the opening faces the second electrode portion while the resin coating contacts the second bonding object, and bonding the first bonding object and the second bonding object by heat-treatment which causes the first electrode portion and the second electrode portion to be electrically connected with each other via the metal while causing the resin coating to harden.
Such a process of making an electrode-to-electrode bond structure is suitable for high-density mounting with a reduced number of process steps in bonding and electrically connecting a semiconductor chip to a semiconductor chip, in mounting a semiconductor chip on a wiring board, and in connecting a wiring board to a wiring board, while achieving a sufficient reliability of the bond.
According to the first aspect of the present invention, no bump is formed on the first bonding object in the step of placing the first bonding object in facing relationship to the second bonding object. There is no need, therefore, to apply flux to the second bonding object for removing the oxide coating from the bump surfaces and for preventing reoxidation of the bump surfaces. Further, since the alignment is performed via the viscous metal paste filled in the openings of the resin coating, there is no need either to apply flux for provisionally fixing the first bonding object to the second bonding object. Since no flux is used in the step of placing the first bonding object relative to the second bonding object in an appropriate orientation, even if there is only a small spacing between the first bonding object and the second bonding object, there is no troublesome step of washing the flux away.
Further, the resin coating hardens when the metal in the metal paste is melted, whereby the first bonding object and the second bonding object are bonded together by the resin coating. Therefore, even if there is only a small spacing between the first bonding object and the second bonding object, it is possible to bond the two objects together by the intervening resin coating which is placed in between in advance.
As described above, since there is no need for removing the flux from and filling the under-fill resin to between the first bonding object and the second bonding object, it becomes possible to provide electrodes on the first and the second bonding objects at a fine pitch. It is also possible to reduce the spacing between the first bonding object and the second bonding object to no greater than 50 xcexcm. Thus, the present invention is suitable for high-density mounting.
Further, according to the present invention, there is no need for coating and removing a flux, and for filling an under-fill resin. Therefore, the number of process steps is reduced in comparison with the conventional process.
A liquid fluxing under-fill resin, which has been conventionally used, may remain at bump-to-electrode interfaces, thus deteriorating the bump-to-electrode connections. According to the present invention, on the contrary, the resin coating does not enter between the electrode portion and the metal paste. Thus, even if the inorganic filler is added at a proportion of 20 wt % or more for regulating the thermal expansion of the resin coating, the filler does not cause an initial conduction failure due to improper electrical connection. Therefore, a sufficient amount of the inorganic filler may be added for achieving a sufficient bonding reliability in the electrode-to-electrode bond structure.
In a preferred embodiment, the metal is a solder powder which melts in the bonding step. Preferably, the metal paste contains a resin component which hardens in the bonding step. Further, the resin coating should preferably soften at a temperature not higher than a melting point of the metal.
In another preferred embodiment, the metal comprises Ag or Cu, and the metal paste contains a resin component which is allowed to harden in the bonding step without melting of the metal. In this embodiment, the resin coating should preferably soften at a temperature not higher than a hardening temperature of the resin component.
Preferably, the metal has a melting point of 80-380xc2x0 C.
Preferably, the resin coating is photosensitive.
Preferably, the resin coating is provided by a film.
Preferably, the metal is contained in the metal paste at a proportion of 30-70 vol %.
Preferably, the resin component and the resin coating contain a same main resin ingredient. In this case, the resin component and the resin coating are integrated with each other in the bonding step.
Alternatively, the resin coating contains a main resin ingredient, whereas the resin component contains a hardener for hardening the main resin ingredient.
Conversely, the resin component may contain a main resin ingredient, whereas the resin coating may contain a hardener for hardening the main resin ingredient.
Preferably, the resin coating contains an inorganic filler at a proportion of 30-70 wt %.
Preferably, the bonding step may comprise pressing one of the first bonding object and the second bonding object against the other of the first bonding object and the second bonding object.
According to a second aspect of the present invention, another process is provided for making an electrode-to-electrode bond structure. The process comprises the steps of forming a resin coating on a first bonding object having a first electrode portion in a manner such that the resin coating covers the first electrode portion, forming an opening in the resin coating to expose the first electrode portion, forming a conductor in the opening, placing the first bonding object relative to a second bonding object having a second electrode portion in a manner such that the second electrode portion faces the conductor while the resin coating contacts the second bonding object, and bonding the first bonding object and the second bonding object by heat-treatment which causes the first electrode portion and the second electrode portion to be electrically connected with each other via the conductor while causing the resin coating to harden.
Like the process according to the first aspect of the present invention, the process according to the second aspect does not require removal of flux from the gap between the first bonding object and the second bonding object, nor supply of under-fill resin into the gap. Therefore, the process according to the second aspect enjoys the same advantages (high density mounting, high bond reliability and reduction of the process steps) as the process according to the first aspect.
Preferably, the conductor is melted for fusion to the first electrode portion and/or the second electrode portion in the bonding step.
Preferably, the conductor is formed by electroplating and/or electroless plating.
Preferably, the conductor has a laminate structure having a plurality of layers each made of a different metal.
Preferably, at least a part of the conductor has a melting point of 80-400xc2x0 C.
Preferably, the resin coating is photosensitive.
Preferably, the resin coating is provided by a film.
Preferably, the resin coating contains an inorganic filler at a proportion of 30-70 wt %.
Preferably, the bonding step comprises pressing one of the first bonding object and the second bonding object against the other of the first bonding object and the second bonding object.
According to a third aspect of the present invention, another process is provided for making an electrode-to-electrode bond structure. The process comprises the steps of forming a resin coating on a first bonding object having a first electrode portion in a manner such that the resin coating covers the first electrode portion, forming an opening in the resin coating to expose the first electrode portion, filling the opening with a bump forming material containing a metal, forming a bump at the opening by heating, placing the first bonding object relative to a second bonding object having a second electrode portion in a manner such that the second electrode portion faces the bump while the resin coating contacts the second bonding object, and bonding the first bonding object and the second bonding object by heat-treatment which causes the first electrode portion and the second electrode portion to be electrically connected with each other via the metal while causing the resin coating to harden.
The process according to the third aspect is suitable for high-density mounting with a reduced number of process steps while also being capable of achieving a sufficient bonding reliability, for the same reasons as described above for the first aspect of the present invention.
A fourth aspect of the present invention provides an electrode-to-electrode bond structure formed by either one of the above-described processes.
According to a fifth aspect of the present invention, a process is provided for connecting a first bonding object and a second bonding object, wherein the first bonding object is provided with a first electrode portion and a resin coating which has an opening for exposing the first electrode portion but otherwise covers the first bonding object, and wherein the second bonding object is provided with a second electrode corresponding to the first electrode portion. The process comprises the steps of filling the opening with a metal paste containing a metal, placing the first bonding object relative to a second bonding object in a manner such that the first electrode portion faces the second electrode portion while the resin coating contacts the second bonding object, and bonding the first bonding object and the second bonding object by heat-treatment which causes the first electrode portion and the second electrode portion to be electrically connected with each other via the metal while causing the resin coating to harden.
According to a sixth aspect of the present invention, a process is provided for connecting a first bonding object and a second bonding object, wherein the first bonding object is provided with a first electrode portion and a resin coating which has an opening for exposing the first electrode portion but otherwise covers the first bonding object, and wherein the second bonding object is provided with a second electrode corresponding to the first electrode portion. The process comprises the steps of forming a conductor in the opening, placing the first bonding object relative to the second bonding object in a manner such that the first electrode portion faces the conductor while the resin coating contacts the second bonding object, and bonding the first bonding object and the second bonding object by heat-treatment which causes the first electrode portion and the second electrode portion to be electrically connected with each other via the conductor while causing the resin coating to harden.
According to a seventh aspect of the present invention, a process is provided for connecting a first bonding object and a second bonding object, wherein the first bonding object is provided with a first electrode portion and a resin coating which has an opening for exposing the first electrode portion but otherwise covers the first bonding object, and wherein the second bonding object is provided with a second electrode corresponding to the first electrode portion. The process comprises the steps of filling the opening with a bump forming material containing a metal, forming bumps at the opening by heating, placing the first bonding object relative to the second bonding object in a manner such that the second electrode portion faces the bump while the resin coating contacts the second bonding object, and bonding the first bonding object and the second bonding object by heat-treatment which causes the first electrode portion and the second electrode portion to be electrically connected with each other via the bump while causing the resin coating to harden.
Like the process according to the first aspect of the present invention, the process according to each of the fifth to the seventh aspects does not require removal of flux from the gap between the first bonding object and the second bonding object, nor supply of under-fill resin into the gap. Therefore, the process according to each of these aspects enjoys the same advantages (high density mounting, high bond reliability and reduction of the process steps) as the process according to the first aspect.
According to an eighth aspect of the present invention, a process is provided for preparing an intermediate product used for making an electrode-to-electrode bond structure. The process comprises the steps of forming a resin coating on a first bonding object having a first electrode portion in a manner such that the resin coating covers the first electrode portion, forming an opening in the resin coating to expose the first electrode portion, and forming a conductor in the opening, wherein the resin coating is hardenable by heating.
According to a ninth aspect of the present invention, a process is provided for preparing another intermediate product used for making an electrode-to-electrode bond structure. The process comprises the steps of forming a resin coating on a first bonding object having a first electrode portion in a manner such that the resin coating covers the first electrode portion, forming an opening in the resin coating to expose the first electrode portion, and filling the opening with a bump forming material containing a metal, wherein the resin coating is hardenable by heating for re-flow of the bump forming material.
A tenth aspect of the present invention provides an intermediate product formed by the above-described process of preparing such an intermediate product.
According to an eleventh aspect of the present invention, an electrode-to-electrode bond structure is provided which comprises a first bonding object having a first electrode portion, a second bonding object having a second electrode portion facing the first electrode portion, an electric conductor having a intermediate constricted portion for connecting the first electrode portion and the second electrode portion, and a sealing resin sealing a gap between the first bonding object and the second bonding object.
Preferably, the sealing resin contains an inorganic filler at a proportion of 30-70 wt %. Further, each of the first bonding and the second bonding object may be either a semiconductor chip or a wiring board.
Other objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments given with reference to the accompanying drawings.